After yesterday's attack, large groups of people were seen streaming out of the centre, some of whom were covered in blood and clutching small children.

Shopping centre guards used trolleys to wheel out several wounded children and at least one man.

Hannah Chisholm, a Briton visiting Nairobi, said she and 60 others barricaded themselves into a large storeroom.

She told the BBC: "We kept running to different places but the shots were getting louder so we barricaded ourselves along with about 60 others into a large storeroom. There were children hiding with us as well as someone who had been shot."

She added: "The gunfire was loud and we were scared but at that point we thought the gunmen were thieves so we assumed they wouldn't try to reach the storeroom."

The volleys of gunfire moved outside the centre after police arrived nearly half-an-hour after the attacks began and engaged the gunmen.

Sporadic gun shots could be heard hours after the assault started as soldiers surrounded the mall and police combed the building, hunting down the attackers shop by shop.

Police helicopters circled above the mall as armed police shouted "get out, get out", and scores of shoppers fled the building. Smoke poured out of one entrance and witnesses said they heard grenade blasts.

Others said they saw about five armed assailants storm the Westgate shopping mall and that the incident appeared to be an attack rather than an armed robbery.

One eyewitness who identified himself as Taha said he heard the screech of brakes followed moments later by an explosion and then sustained gun fire from the ground floor.

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Another survivor said he was shot by a man who looked Somali.

Some shoppers ran upstairs and escalators and hid around the mall’s cinema complex. Police found another terrified group hiding in a toilet on the first floor.

At least two dozen wounded were wheeled out on stretchers and shopping trolleys. Many of the victims had multiple light wounds, apparently from flying debris. Other walked out, some with bloodied clothing wrapped around wounds.

Al Shabaab have previously threatened to launch strikes on Nairobi’s tower blocks and soft targets including nightclubs and hotels known to be popular with Westerners in the capital. But they have so far failed to carry out such an attack.

"I personally touched the eyes of four people and they were dead. One of them was a child," said one former British soldier at the scene.

"It’s carnage up there."

A policeman patrols with his dog (Image: Reuters)

Police cordoned off the roads surrounding the mall in central Nairobi’s Westlands neighbourhood.

Satpal Singh, who was in another cafe on the mall’s top floor said he ran downstairs when he heard the gunfire and was shot at near the mall’s main exit.

"A Somali guy shot at me. The guy who shot me was carrying a rifle, an AK-47," 36-year-old Singh said.

The Foreign Office updated the travel advice on its website to say: "British nationals should avoid the area".

Concerned British nationals are advised to monitor FCO travel advice and to contact 020 7008 0000.